Blue clay almost turned white (3,4)
I believe the answer is:
cue ball
'white' is the definition.
(the cue ball in snooker is also called the white)
'blue clay almost turned' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'turned' indicates anagramming the letters.
'blue'+'clay'='blueclay'
'blueclay' with its last letter taken away is 'bluecla'.
'bluecla' is an anagram of 'CUE-BALL'.
(Other definitions for cue ball that I've seen before include "Jimmy White's white?" , "The white in snooker" , "Object struck by a snooker player" , "Feature of pub entertainment" .)